Propylene oxide (PO) is very important organic chemical material, which ranks only next to polypropylene and acrylonitrile in yield among propylene derivatives. Propylene oxide is mainly used for preparing polyether, propylene glycol, isopropanolamine, non-polyether polyhydric alcohols, and the like, thereby further producing unsaturated polyester resin, polyurethane, surface active agent, and fire retardant, and so on. Propylene oxide is widely used in the industries of chemical engineering, light manufacturing, pharmaceuticals, food, and textile, and has far-reaching influence on the development of chemical industry as well as the national economy. As the range of application of propylene oxide expands and the use amount of downstream product thereof increases, the market demand for propylene oxide is becoming higher and higher.
Currently, the main processes for the industrial manufacturing of propylene oxide include chlorohydrin process, co-oxidation process for propylene oxide with co-product (PO/SM process and PO/MTBE process or PO/TBA process), and cumyl hydroperoxide process for propylene oxide without co-product (CHP process). Because the chlorohydrin process produces large amount of chlorine-containing effluent in the manufacturing process, environmental pollution will be caused and the apparatus will be severely corroded. Co-oxidation process for propylene oxide with co-product can eliminate the defects of pollution and corrosion of the chlorohydrin process, but it also suffers from lengthy technological process, large investment, and large amount of co-products, which influence the manufacturing of propylene oxide to a certain extent. CHP process has become the development trend for the production technology of propylene oxide due to its light pollution level and free of co-product.
The technology for preparing propylene oxide compound from cumyl hydroperoxide (CHP) and propylene in the presence of fixed bed catalysts is known. Said technology mainly comprises three reaction steps. First, atmospheric oxidation of cumene takes place for preparing cumyl hydroperoxide. Then, an epoxidation reaction between CHP and propylene in the presence of heterogeneous catalysts takes place, and propylene oxide (PO) and α,α-dimethyl-benzyl alcohol (DMBA) are generated. Subsequently, hydrogenolysis reaction of DMBA with H2 takes place in the presence of catalysts, and generates cumene, which is recirculated to the oxidation process for preparing CHP. In order to improve the conversion ratio of CHP, excessive amount of propylene is usually used. For example, the molar ratio of propylene to CHP is in a range of 5-20, thus there is excessive amount of propylene in the reaction product. In order to improve the epoxidation efficiency and reduce the load of PO refinement, it is required that the propylene in the reaction product be recycled. The circulating propylene should have high purity and be rid of impurities. In the meantime, the accumulation of inert components in the circulating system should be avoided.
According to literature CN1505616A, a process for preparing propylene oxide is proposed, comprising the following steps: first, propylene is reacted with cumyl hydroperoxide in the presence of catalysts, and propylene oxide is generated; then a reaction mixture obtained from the first step is distilled, and unreacted propylene is recycled from the distillation column. A bottom temperature of the distillation column is set at 200° C. or lower. According to the above process, crude PO product is obtained from the bottom of the distillation column and propylene is obtained from the top thereof. Due to the thermo sensitivity of PO, the temperature of the column bottom is generally controlled no higher than 130° C. during industrial production. That is, the operating pressure of the distillation column is defined, rendering the operating temperature at the top of the distillation column to be lower than 40° C. As a result, it is impossible to use conventional cooling water as the cryogen, but rather, large amount of cryogen of even lower temperature is required for the condensation recovery of propylene. Consequently, the industrial operation will be difficult and the power consumption will be high.